For some time the state of the art has included screening devices to separate and select chips and particles according to their gradings from a loose or fibrous mass, whether it be dry or wet, generally but not exclusively of wood-based material.
Although the state of the art covers separating devices incorporating rollers, comprising adjacent rollers which rotate in the same direction and which define a bed on which the material to be selected is fed, in the past, and in the industrial field there was a large-scale preference to use screens of the type with a vibrating or oscillating netting, or also with rotary disks.
Within the range of these devices, screens with netting or with disks have been developed with one or more levels or orders of selecting elements; screens with netting or with disks using respectively meshes or gaps where the passage area is progressively increased, or also screens with netting or a disk with transverse bands, where the opening distance is progressively increased in order to discharge particles with a progressively increasing grading.
It is only in recent times that the use of screens and forming machines incorporating rollers has become of considerable importance in the industrial field, especially since materials which are highly resistant to wear, such as special steels, high resistance linings, etc., have become available at a reasonable cost.
The availability of these materials with very high surface resistance and hardness has made it possible, in recent years, to produce and employ separating devices with rollers, which have a great efficiency of production, a long life, and limited or no maintenance; this has made the applications of these machines, both simple screens on one or more levels, and also forming machines, extremely advantageous.
Although the technology of roller screens has been known to the state of the art for many years, it is only in recent years that it has found a real, large-scale industrial application, for the reasons given above.
In the light of these developments, linked to the increasingly evolved types of material available, there have been trials and experimentations in the field on solutions which substantially reproduce the effects and the functions of screens with netting and with disks, though their efficiency has been increased, thanks to the natural functionality of roller devices.
The natural functionality of roller devices is shown particularly in the selection of the fine particles, since the use of rollers instead of, for example, disks mounted on disk-bearing shafts, makes it possible to accurately gauge the gaps to an extremely reduced size which is both continuous and constant.
When disks are used, in fact, the discharge gap is of a substantially rectangular shape, where the distance between the surfaces of the adjacent disk-bearing shafts determines the length of the particles to be selected, while the distance between two adjacent disks mounted on the same shaft determines the thickness of the particles to be selected.
It should however be noted that neither netting screens nor disk screens normally allow the discharge gap in the individual sections of the selection bed to be varied during the operating cycle.
The natural functionality of roller devices has the following direct, resulting advantages:
the preferential choice of the reliefs on the surfaces of the rollers in order to obtain a more coherent screening with the chips, particles and fibres available, and with the specific desired result; PA1 the possibility to distance the rollers reciprocally, both on the horizontal and vertical plane, so as to vary the discharge gap even during the operating cycle, and also to adjust, if so required, the speed of rotation of the rollers, also during the operating cycle.
These features have been the object of a multitude of patents, filed at different times and at long intervals, and therefore their solutions must be considered according to what technology was available at that specific time, with particular reference to the materials available and usable in the industrial field.
For example, U.S. Pat. No. 1,424, published in 1839, already discloses a separating device, in this case for the screening of lead oxide, including adjacent rollers equipped with grooves and penetrating peaks.
U.S. Pat. No. 292,656 also discloses rollers, with mating V-shaped threads on the surface, with a sloping and substantially helical development.
However, this embodiment has the disadvantage that it progressively displaces the material to be selected in a transverse direction with respect to the direction of feed.
U.S. Pat. No. 1,173,737, published in 1916, includes a screener with parallel rollers where the rollers include grooves cooperating with the mating tapered peaks of the adjacent rollers, and where the grooves are not penetrated by the peaks but together define a constant gap through which the particles can pass, the gap being substantially perpendicular to the direction of feed of the material.
U.S. Pat. No. 4,452,694 describes a selection device consisting of a plurality of disks arranged in a line in a plurality of parallel axes forming all together a conveyor bed for the material to be selected, the material being transported in a direction substantially at a right angle to the axis of the disks.
This conveyor bed includes a feeder end on one side and a discharge end at the other.
According to this document, the peripheral surface of the disks includes disks with protrusions or tapered peaks which position themselves in a mating position with tapered recesses or grooves on the adjacent disks.
According to this document, moreover, the rotary speeds of the disk-bearing shafts can be different.
WO 86/01580 refers to selection devices used in incinerator plants. It uses rollers which have on their surfaces protruding ribs with a development substantially parallel to the axis of the relative roller; the walls of the ribs are substantially perpendicular to the roller.
The ribs of one roller face the ribs of the other roller, but do not mutually penetrate each other, so that the discharge gap is substantially linear.
U.S. Pat. No. 3,387,795 discloses a device to process fibrous material comprising a plurality of adjacent rollers which include on their circumferential surface pyramid-shaped tapered protrusions, separated by tapered grooves, the protrusions penetrating at least partly into the tapered grooves of the adjacent roller.
EP-B-328.067 discloses a roller device where the outer circumferential surface of the rollers has individual tapered pyramid-shaped protuberances, developing substantially in a spiral around the surface of the rollers and extending lengthwise, separated by tapered grooves.
The tips of the protuberances of two adjacent rollers face one another and define the discharge gap for the selected material; therefore, there is no penetration of the grooves by the protuberances. The discharge gap is substantially constant, at a right angle to the direction of feed of the material, and parallel to the axis of the rollers.
This embodiment, compared with the afore-mentioned U.S. Pat. No. 1,173,737 substantially includes the sole characteristic that its protuberances are pyramid-shaped and tapered, and this characteristic is in any case included in the afore-mentioned U.S. Pat. No. 3,387,795.
The modifications to the surfaces of the rollers and the disks make it possible to reproduce the natural and intrinsic effect of the netting screens and forming machines on the chips, thus obtaining a good decantation of the finer particles.
Another function of these surface modifications is to delay the passage of the cubic particles through the discharge gap as a result of the dynamic thrust caused by the faces of the protrusions of the counter-opposed rollers.
Moreover, the inclusion of these surface modifications brings the advantage that they do not cause the rollers to jam when the device starts off again; in effect, this makes the roller devices comparable to the netting screens where no problems are caused when the screening is stopped.
The conformation of the surface modifications known to the state of the art, together with a discharge gap at right angles to the direction of feed of the material, proved to have, when used, a plurality of disadvantages which had not been foreseen.
To be more exact, it has been seen that with rollers of the type known to the state of the art the passage of long and light fibrous particles through the discharge gap is very difficult.
In fact, it is very difficult for fibrous particles which are much longer than the discharge gap to pass, even if they are less thick than the discharge gap, as these long particles tend to form a bridge and therefore are not discharged through the gap.
As a result, these particles are only discharged when the gaps are much thicker than the particles themselves, and consequently also discharge short particles and cubic particles of an undesired thickness, that is to say, excessively thick.
This is an extremely serious problem for the subsequent use of the particles and substantially compromises, in many cases, the possibility of using this type of screen, particularly in forming machines, since this widening of the gap leads to cubic particles being accepted, and the latter cause an "orange peel" effect on the surface layers of the mat.
For some time the process has been known to the state of the art, namely from U.S. Pat. Nos. 3,848,741, 4,209,097, CA-A-651.347, whereby the rollers are positioned on the horizontal plane and maintained parallel, in order to vary the gap between adjacent rollers.
DE-C-2.358.022 and SU-A-1.227.263 disclose how to move one roller in alternation to the adjacent roller on the vertical plane in order to vary the gap to discharge the material.
The state of the art also covers the fact that the speed of rotation of the rollers may be adjusted.
DE-A-95 874 refers to a roller-type sizing device for materials in particle form, specifically for coal particles.
The rollers are peripherally equipped with alternate peaks and grooves, wherein the peaks of one roller face the grooves on the adjacent roller.
The peaks and grooves may be rounded, segmented or with a sharp edge.
It is also possible that the surfaces of the peaks and grooves may have channels.
This document refers to the sizing of materials which, once crumbled, take on a shape substantially of little cubes or similar, according to what is said in the first part of the description.
Moreover, the description says that among these materials a flat shape is never found.
On the contrary, due to the fibrous nature of the material, wood chips and particles tend to have an elongated shape, with a thickness much less than their length and width.
With materials derived from wood, therefore, it is usual to find a flat shape, in fact it is the most frequent shape.
Particles which are thin and have elongated fibres are more valuable, and they must be sized in a most rigorous manner.
A device such as that described in DE'874, if applied to size wood chips and particles, would not carry out its function efficiently.
In fact, as explained before, particles of wood material which are flat, fibrous and light, and much greater in length than the discharge gap, tend to form a bridge and are not discharged through the discharge gap even if their thickness is less than the width of the said gap.
Because of this shortcoming, cube-shaped particles of a greater and therefore unacceptable thickness are also accepted, together with the long fibrous particles.
Moreover, the substantially flat shape of the surface of the rollers disclosed in DE'874, even if they have channels, causes only a partial vibration of the material which is not at all sufficient for long, flat particles like wood particles, even though it may be sufficient for large, heavy particles, square or cube like particles of coal.
Furthermore, the penetration of the grooves by the peaks is only partial and limited, and not even this characteristic is suitable if referred to sizing wood-based chips or particles.
GB-A-280,191 also refers to a sizing device for particles of coal or similar, and has the same disadvantages as DE'874.
Even if the discharge gap defined by the disks is in a zig-zag, this zig-zag development is obtained with large variations of section--a phenomenon which is not very suitable to select fibrous particles and wood chips correctly and in a uniform manner.
Moreover, the protrusions on the disks perform a cutting function, which may not damage heavy, cubic particles, but considerably damages long, light particles.
Furthermore, this document does not teach to provide a sufficient vibratory effect on the particles which are to be sized, and moreover the mutually penetrating disks do not allow thin particles to be gauged.
The present Applicants, aware of the optimum functioning of screens and forming machines incorporating netting, and considering the shortcomings of the state of the art in screens and forming machines incorporating disks and rollers, have designed, tested and embodied innovative solutions to be applied to roller devices, whether they be employed with the sole function of screening, with one or more beds of rollers arranged in sequence or on several planes with a constant or progressively increasing discharge gap, and also in possible applications for roller-type forming systems.